<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        /* 求1+2!+3!+...+20!的值 */
        var num = 0
for(a=1;a<21;a++){
   var c=1;
for(b=1;b<=a;b++){
  c*=b;

}
num=num+c
}
console.log(num)



    </script>
</body>
</html>